Structure & Function of the Mammalian Heart

Bio FactsheetJanuary 1999 Number 35

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The structure of the heartThe structure of the heart is illustrated in Figs 1a and b. The structure isclosely related to its function.

This Factsheet summarises:l. The structure of the heart and the histology of cardiac muscle.2. The cardiac cycle and its regulation by the heart itself.3. The modification of the cardiac cycle by nerves and hormones.

Fig 1a. External structure of the heart, ventral view

Right pulmonary artery(to right lung)

Superior vena cava(from head andneck)

Right atrium

Inferior vena cava(from body)

Right ventricle

Dorsal aorta(from aortic arch, to body)

Aortic arch (to body)

Left pulmonary artery(to left lung)

Pulmonary arch(to lungs)

Pulmonary veins(two of four,

from lungs)

Left atrium

Left ventricle

Coronary arteries

via the anterior and posterior venae cavae. This blood is pumped by theright ventricle to the lungs via the pulmonary arch and pulmonary arteries.The left atrium receives oxygenated blood from the lungs via the pulmonaryveins. This blood is pumped by the left ventricle to the body via the aorticarch and aorta.

There are valves in the heart and in the veins to maintain a linear flow ofblood, i.e. in one direction only. In the bases of the aortic and pulmonaryarches are the semilunar valves which prevent backflow from arches toventricles. Between each atrium and ventricle is an atrioventricular valvewhich prevents backflow from ventricle to atrium. The tricuspid valve isbetween the two right chambers and the bicuspid (mitral) valve is betweenthe two left chambers. The atrioventricular valves are held open or closedby tendons (chordae tendinae or heart strings) and are regulated by thepapillary muscles of the ventricle wall to which the tendons are attached.It is the slamming shut of the heart valves that results in the typical ‘lub-dub’ sound that is heard when listening through a stethoscope.

Structure: FunctionThe walls and septum of the heart are composed of cardiac muscle andtheir thickness is closely related to the pumping requirement of each chamber(Table 1).

Table 1. Thickness of cardiac walls

Cardiac muscle has a superb blood supply of its own called the coronarycirculation . This ensures that the muscle receives adequate nutrients andoxygen for its continual functioning and that waste products are removedefficiently. The human heart beats on average 72 times every minutethroughout life and so the coronary circulation has to be super-efficient.The cardiac muscle is arranged as a ‘basin shaped’ mass of tissue aroundeach heart chamber, and so contraction of its cells results in a decreasedchamber volume thus forcing blood onwards.

The histology of cardiac muscleThis is illustrated in Fig 2 overleaf. The contractile proteins of cardiacmuscle are arranged into sarcomeres, fibrils and fibres as in voluntarystriated muscle, but in cardiac muscle the fibres are arranged into branched,short individual cells with elaborate junctions (intercalated discs) betweenthem. These discs are highly folded interlocking areas of the cell membrane(sarcolemma) consisting of areas of adhesion to hold the cells together in asheet formation, and of areas where rapid ion transport can occur from cellto cell, thus allowing waves of depolarisation (contraction) and repolarisation(relaxation) to pass through the tissue. Between the cells are numerouscapillaries of the coronary circulation. Within the cells are many mitochondria

Fig 1b. Internal structure of the heart, ventral view

Aortic arch

Pulmonary arch

Superior vena cava

Right atrium

Tricuspid valve

Valve tendon

Papillary muscle

Inferior vena cava

Pulmonary and Aorticsemilunar valves

Pulmonary veins

Left atrium

Bicuspid valve

Left ventricle

Ventricular septum

Mammals have a double circulation; the pulmonary circulation carriesblood to and from the lungs and the systemic circulation carries blood toand from the rest of the body. The heart thus acts as a double pump inwhich the two sides are completely separated by a wall (septum) (Fig 1).The right side pumps deoxygenated blood to the lungs for gas exchangewith the alveolar air, while the left side pumps oxygenated blood to thebody for gas exchange with the tissues. The heart has four chambers, theright and left atria and the right and left ventricles. The atria pass blood tothe ventricles. The right atrium receives deoxygenated blood from the body

Cardiac Wall

Atrial walls andatrial septum

Right ventricle walland ventricularseptum

Left ventricle wall

Thickness

Fairly thin

Thick

Extremelythick

Function (reason for thickness)

Pumps blood to ventricles= short distance

Pumps blood through lungs= medium distance

Pumps blood throughout body = very long distance

Structure & Function of the Mammalian Heart Bio Factsheet

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Fig 2. Histology of cardiac muscle tissue.Light microscopic features

Intercalated disc, dividing fibre intocells

Nucleus, placed centrally in cell

Branched muscle fibre (under thelight microscope this would have asimilar striated appearance tovoluntary muscle)

Spaces between cells filled withhighly vascular loose connectivetissue (coronary blood supply)

enabling the tissue to have a very high metabolic rate for long periods,when needed. The tissue is said to be ‘myogenic’ which means that it willcontract and relax rhythmically of its own accord throughout life. Thenervous system is only involved in altering the frequency and force ofthese contractions to meet body needs.

The sheets of cardiac muscle making up the walls of the heart are firmlyanchored to a tough ring of collagen fibres which makes up the skeleton ofthe heart. This is situated in the wall of the heart between the atria and theventricles, and runs round the full circumference of the heart in this region.

The cardiac cycleThe pumping activity of the heart is described as the cardiac cycle. Onecardiac cycle consists of systole (contraction) and diastole (relaxation).The two ventricles contract simultaneously from the apex upwards but theatria contract a fraction of a second before the ventricles, the right atriumcontracting slightly before the left atrium. This sequencing of contractions,coupled with the actions of the valves, ensures a linear flow of blood whichwould not occur if all parts of the heart contracted and relaxedsimultaneously. As long as the heart is pumping effectively there should beno damming up of blood, and the cardiac output from the ventricles shouldequal the venous return to the atria. The stages of the cardiac cycle are asfollows:

1. Initially the heart is in diastole with atria and ventricles relaxed, theatrioventricular valves are open, but the arch semilunar valves are shut.

2. The venous return of deoxygenated blood from the body enters therelaxing right atrium through the venae cavae.

3. The venous return of oxygenated blood from the lungs enters the relaxingleft atrium through the pulmonary veins. At this stage some blood alsopasses through the open atrioventricular valves into the relaxingventricles.

4. When the atria are full, their walls contract (atrial systole) pushingblood through the open atrioventricular valves into the relaxed ventricles.Blood cannot pass back into the veins because they contain valveswhich prevent backflow, and also the contracting of the atrial wallspartly closes off the entries of the venae cavae and pulmonary veins tothe atria.

5. After a short delay the ventricles contract from the apex upwards,forcing the blood at high pressure into the arches. At this time theatrioventricular valves slam shut, preventing backflow of blood to theatria, and the arch semilunar valves open to allow the passage of bloodinto the arches.

6. When the ventricles relax (diastole) the sudden fall in pressure causesthe arch semilunar valves to slam shut, thus preventing backflow ofblood from the arches.

The cardiac output is the volume of blood pumped out by the heart in dm3

per minute. In humans, at rest, this averages around 5 dm3 min-1 and canrise to around 20dm3 min-l during severe exercise. The ‘stroke volume’ ofthe heart is the volume of blood forced out per beat, and can be calculatedby dividing the cardiac output by the beat frequency per minute. Forinstance, at rest, with a cardiac output of 5 dm3 min-1 and a beat frequencyof 70 beats per minute, the stroke volume would work out to a value of71.4 cm3 per beat.

Regulation of the cardiac cycle by the heart itselfOwing to the myogenic nature of cardiac muscle, the heart beats of its ownaccord in a co-ordinated sequential pattern, even if the nervous connectionsto the heart are severed, for instance, in transplant surgery. Due to ionexchange across the intercalated discs, the contraction of one cell willstimulate the contraction of other cells at either end. Thus, the cell with thefastest rate of spontaneous contraction will impose its rate on thesurrounding cells.

The region of the heart that has the highest inherent rhythm is called thesino-atrial node (SAN or pacemaker) which is situated in the wall of theright atrium between the entries of the superior and inferior venae cavae.This node initiates waves of depolarisation (which cause contraction) whichspread first through the wall of the right atrium and then through the wallof the left atrium. This is why the right atrium always contracts slightlybefore the left atrium. The waves of depolarisation cannot pass throughthe heart wall from atria to ventricles because of the high electrical resistanceof the ring of collagen making up the heart skeleton, but can only pass viaa region of conducting tissue, called the atrioventricular node (AVN) ,which is situated at the top of the ventricular septum (Fig 4)

Fig 3. The heart at different stages of the cardiac cycle

Ventricular systole(Ventricles contract)

Diastole (atria andventricles relax)

Atrial systole(atria contract,ventricles relaxed)

RA

RV

LA

LVRA

RV

LA

LV

RA

RV

LA

LV

Fig 3 illustrates the cardiac cycle.

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From here the depolarisation impulses are passed rapidly to the apex ofthe heart through specialised conducting cells, called Purkinje fibres(sometimes spelt Purkyne). These fibres make up the left and rightventricular bundles, which together are called the Bundle of His. From theapex, the waves of depolarisation pass through the cardiac muscle fibres ofthe ventricles, therefore causing contraction to occur from apex upwards.

When the impulses pass through the atrioventricular node, they are delayedfor about 1/10th of a second. This ensures that ventricular contractionoccurs after atrial contraction. As the cardiac muscles repolarise back tothe resting state, they will relax. The nodes, bundle of His and Purkinjefibres make up the conducting system of the heart. The above processesare summarised in Fig 5.

Modification of the cardiac cycle

1. By hormonesAdrenaline and noradrenaline, produced by the adrenal medulla,increase the rate and force of cardiac muscle contractions, and alsobring about dilation of the coronary arterioles thus improving the bloodsupply to the heart muscle. The actions of these hormones iscounteracted by the actions of acetylcholine from the parasympatheticsystem (PNS).

2. By the nervous systemAlthough the cardiac cycle is initiated and co-ordinated by the heartitself, the cardiac output must be adjusted to meet the varying needs ofthe body, and this involves the use of sensory receptors and theautonomic nervous system. Stimulation of the heart by the sympatheticnervous system (SNS) increases cardiac output, by:(i) increasing the frequency of the heartbeat, and(ii) increasing the force with which the cardiac muscle contracts.

Stimulation of the heart by the parasympathetic nervous system reducesthe cardiac output by reducing the beat frequency and force of contraction.

These two systems operate through negative feedback control involvingtwo controlling centres in the medulla of the brain. These are thecardioacceleratory centre (CAC) which is concerned with increasingcardiac output and the cardioinhibitory centre (CIC) which reducescardiac output.

From the CAC, sympathetic nerves pass down the spinal cord and exit asaccelerator nerves to the sinotrial node, atrioventricular node and cardiacmuscle fibres, where they release noradrenaline. This stimulates thesinoatrial node to increase its depolarisation frequency, reduces the delayat the atrioventricular node and increases the force of the muscularcontractions. Thus the cardiac output rises.

From the CIC, parasympathetic fibres pass through the vagus (10th cranial)nerves to the sinotrial node and the atrioventricular node, where theyrelease acetylcholine. This suppresses the activity of the sinoatrial nodeand increases the delay at the atrioventricular node. Thus the cardiac outputfalls (Table 2).

Fig 4. Internal view of heart to show the conducting system

Fig 5. Nervous stimulation of the heart

SAN Wall of R.A. Wall of L.A.

AVN

Purkinje fibres(Bundle of His)

Apex

viac.m

viac.m

Stimulation or inhibition of the cardiac control centres involves threeseparate reflex pathways which are associated with blood pressure receptors(baroreceptors) situated in different positions.

The reflexes are:• the carotid sinus reflex• the aortic reflex• the Bainbridge reflex.

The carotid sinus reflex is concerned with maintaining the correct bloodsupply to the brain. The carotid sinus is a swelling on the base of theinternal carotid artery to the brain and the baroreceptors are in its wall. Ifthe blood pressure rises the sinus wall is stretched and the baroreceptorsfire off impulses to the cardioinhibitory centre, thus cardiac output isreduced and blood pressure falls. As the sinus wall becomes less stretched,the baroreceptors are no longer stimulated and the cardioacceloratory centrenow dominates, thus raising cardiac output and blood pressure.

The aortic reflex is concerned with maintaining the correct blood pressurein the aorta and general circulation. Aortic bodies in the wall of the aorticarch contain the baroreceptors. The aortic reflex operates basically in thesame way as the carotid sinus reflex.

The Bainbridge reflex regulates venous pressure. Baroreceptors in the wallsof the venae cavae and right atrium fire of impulses to the cardio acceleratorycentre thus increasing cardiac output and thus increasing venous return andvenous blood pressure.

Table 2. SNS and PNS control of cardiac output

System

SNSPNS

Transmitter

noradrenalineacetylcholine

Effect

k heart rate & force, k cardiac outputl heart rate & force, l cardiac output

Aortic arch (pulmonary arch omitted)

Right atrium

Sinoatrial node(pacemaker)

Atrioventricularnode (AVN)

Tricuspid valve

Bundle of His

Right ventricular bundle

Purkinje fibres spreading outthrough right ventricle wall

Left atrium

Bicuspid valve

Left ventricle

Left ventricularbundle

Purkinje fibresspreading out

through leftventricle wall

Apex of heart

Exam Hint - Candidates are very rarely asked to draw the heart.Labelling the structure and identifying the nervous pathway of thecardiac cycle is much more commonly asked. When revising,condense the cardiac cycle, the nervous pathways and the action ofthe SNS and PNS into a flow chart.

Structure & Function of the Mammalian Heart Bio Factsheet

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(a) Draw on the diagram the position of the valve and valve tendonsbetween the right atrium and right ventricle while the ventricle iscontracting. (2 marks)

(b) What would happen if the valve was not in the correct position?(1 mark)

(c) (i) Which organs does the blood go to on leaving the right ventricle?(1 mark)

(ii) What changes occur in the blood when it reaches these organs? (3 marks)

4. The heart beat rate of a student was measured before and after a periodof 2 minutes exercise. Immediately before and after the exercise thestudent rested in an arm chair. The results are presented in the graphbelow.

(a) Identify structures labelled A, B, C and DA ........................................ B.................................................C................................................ D........................................................

(4 marks)

Practice Questions

l. The diagram below shows the external appearance of a human heart.

A

B

D

C

E

(b) State the functions of the part labelled E. (2 marks)

(c) The following pressures occur in the heart during a complete cardiaccycle, 0 mm Hg, 20 mm Hg and 180 mm Hg.Which of these pressures do you think is associated with:Atrial systole ...................Ventricular systole ...................Diastole ................... (3 marks)

2. Read through all of the following passage and then fill in the spaceswith the most appropriate word or words.The cardiac cycle is initiated and controlled by the heart itself. Cardiacmuscle is said to be ......................... since it will contract and relax ofits own accord. The beat is initiated by the .........................which issituated in the wall of the ............... Waves of depolarisation travelthrough the atria causing atrial .............. The waves of depolarisationcan only travel to the ventricles via the ................... situated at the topof the ventricular septum. From here the waves travel to the apex ofthe heart through the ..................... which is made of specialisedconducting cells called ........................... These then carry the waves ofdepolarisation through the ventricle walls causing both ventricles tocontract simultaneously. At this stage the ..................... are open andthe ........................... are shut so that blood can be forced into thearches.

(9 marks)

3. The diagram below shows the right side of the heart during ventricularsystole.

Right atrium

Right ventricle wall

(a) (i) By how many beats per minute did the heart rate increase duringthe exercise period? (1 mark)

(ii) How long did it take after the exercise for the heart rate to returnback to the resting level? (1 mark)

(b) (i) Describe the physiological mechanisms involved which caused theincrease in heart rate during the exercise.

(ii) Explain why the return to the resting rate was only gradual. (2 marks)

5. For each of the following features associated with the cardiac cycle,describe its location, and state one of its functions.

140

130

120

110

100

90

80

70

601 2 3 4 5 6 7 8 9 10 11 120

Hea

rt R

ate/

Bea

ts m

in-1

Time/minutesExercise period

Feature

Carotid body (sinus)

Cardio acceleratorycentre

Purkinje tissue

Coronary capillaries

Papillary muscles

Location Function

(10 marks)

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Acknowledgements;This Factsheet was researched and written by Martin GriffinCurriculum Press, Unit 305B, The Big Peg,120 Vyse Street, Birmingham. B18 6NFBio Factsheets may be copied free of charge by teaching staff or students,provided that their school is a registered subscriber.No part of these Factsheets may be reproduced, stored in a retrieval system,or transmitted, in any other form or by any other means, without the priorpermission of the publisher.ISSN 1351-5136

AnswersSemicolons indicate marking points

1. (a) A = aortic arch;B = right atrium;C = pulmonary veins;D = inferior/posterior vena cava;

(b) Coronary arteries/veins to carry blood to and from cardiac muscle;Provides very efficient oxygen supply to cardiac muscle/veryefficient removal of carbon dioxide;Enabling efficient working/respiration of heart muscle;

(c) Atrial systole 20 mm Hg;Ventricular systole 180 mm Hg;Diastole 0 mm Hg;

2. myogenic;sinoatrial node/pacemaker;right atrium;systole/contraction;atrioventricular node;bundle of His/left + right bundle branches;Purkinje fibres;semilunar valves;atrioventricular valves;

3. (a) Valve flaps closed;tendons from valve flaps to papillary muscles;

(b) If the valve was not closed then the systole blood would be forcedback to the atrium rather than to the lungs for oxygenation;

(c) (i) Lungs;(ii) Offloads CO

2;

thus blood hydrogen carbonate concentration falls;onloads O

2;

reformation of oxyhaemoglobin;

4. (a) (i) 70 beats per minute;(ii) 5 minutes;

(b) (i) Increased respiration leads to a decrease in blood O2 tension/

increase in blood CO2/HCO

2 tension;

sensed by chemoreceptors in aortic/carotid bodies;impulses sent to CAC centre in medulla;this increases sympathetic stimulation of the heart;this increases frequency of depolarisation of the sinoatrialnode which increases heart beat rate;CIC inhibited so parasympathetic stimulation of heart reducedduring this time;

(ii) Blood O2/CO

2/HCO

3 tensions take time to be returned to resting

levels;by ventilation mechanisms/breathing;thus CAC not suppressed for several minutes;

Feature

Carotid body(sinus)

Cardio acceleratorycentre

Purkinje tissue

Coronary capillaries

Papillary muscles

Location

Base ofinternalcarotidarteries;

Medulla(oblongata);

Bundle ofHis/betweencardiac musclefibres inventricles;

Betweencardiac musclefibres;

On inside ofventriclewalls;

Function

Senses blood pressure/O2

/CO2tensions;

Increases cardiac output;

Rapid transmission ofimpulses to ventricularmuscle;

Provide good blood/O2

/glucose supply to muscle/efficient CO

2 removal;

Regulate AV valve openingand closing;

5.